Multitrack slip-ring and brush assembly



' July 4, 1967 J. C DAVIS MULTITRACK SLIP-RING AND BRUSH ASSEMBLY fir/0r,4/79

Filed Feb. 24, 1965 f ik w War/ray United States Patent 1 3,329,923MULTITRACK SLIP-RING AND BRUSH ASSEMBLY James Carl Davis, Blacksburg,Va., assignor to Litton Precision Products, Inc, Beverly Hills, Calif.

Filed Feb. 24, 1965, Ser. No. 434,976 8 Claims. (Cl. 339-2) Thisinvention relates to sliding or rotating contact assemblies. Moreparticularly, it relates to brush and slipring assemblies suitable fortransferring electrical signals or electrical power between relativelyrotating or sliding components such as are employed in precisioninstruments, for example, in gyroscopic guidance systems.

Of major importance in precision instruments, such as the gyroscopicsystems used to perform indicating or control functions on modern highspeed aircraft or ships, is the low-noise transmission of low-levelelectrical signals to and from moving or rotating structures. Asignificant source of unwanted electrical fluctuations or noise in suchapparatus is the variation of the resistance between the contacts inbrush and slip ring assemblies. Related problems arise from inadequatecontact life which necessitates frequent and expensive overhaul andrepair to maintain critical operational tolerances.

Accordingly, it is among the objects of this invention to providesliding electrical contact assemblies capable of reliable low-noiseperformance for relatively extended periods.

These and other objects of the invention are achieved in one particularillustrative embodiment which includes a longitudinally extendingchannel or groove having nonparallel conductive walls and a brushassembly having at least one brush mounted for movement relative to thewalls of the groove. The longitudinal extent of the groove may be alonga straight line, along a circle, or along a path of arbitrary curvature.A portion of the brush is positioned in the groove in sliding electricalcontact with the conductive walls. The portion of the brush in the areaof contact is characterized by an elongate cross-section, and the brushis aligned in the groove with the greater cross-sectional dimensionextending thereacross.

In a preferred illustrative embodiment, the groove is V-shaped and thelarger cross-sectional dimension of the brush in the contact area isgreater than the width of the groove at its bottom but less than thewidth at its top. Thus, the brush rides in the groove somewhat above thebottom and contacts the side walls with a surface of relatively highcurvature in a cross-sectional plane. It is considered mostadvantageous, in embodiments employing more than one brush in a groove,for the larger crosssectional dimension of each brush to differ fromthat of the others. Each brush, therefore, will ride in the groove at adifierent height above the bottom and each will contact the side wallsalong a separate set of traclm.

The objects, features and advantages of the invention will be fullyunderstood from the following more detailed description taken inconjunction with the accompanying drawing, in which FIG. 1 depicts aslip-ring and brush assembly of the prior art;

FIG. 2 shows a sliding contact assembly embodying the principles of theinvention; and

FIG. 3 illustrates a slip-ring and brush assembly in accordance with apreferred embodiment of the invention.

There is shown in FIG. 1 a typical prior art slip-ring and brushassembly comprising a slip-ring 11 having a V-shaped circumferentialgroove 12 with conductive side Walls 13 and 14, and a wire type brush 15of circular crosssection. The diameter of brush 15 is smaller than thewidth of the top but greater than the width of the bottom of groove 12so that the brush rides in the groove somewhat above its bottom and insliding conductive contact with walls 13 and 14.

While the groove type slip-ring of FIG. 1 was a substantial improvementover earlier types in which the round brush rode on a single flatsurface, it is still subject to difficulties which give rise tosubstantial and troublesome variations in the contact resistance atpoints 16 and 17, thereby causing undesirable noise in the electricalcircuits associated with the precision instruments in which they arefrequently used, such as gyroscopic instruments. For example, wearparticles, dust and other foreign particles may accumulate in the bottomof the groove 12 to a depth sulficient to lift the brush intermittentlyout of contact with one or both of the groove walls.

Furthermore, the angle between walls 13 and 14 and the surface of thebrush 15 adjacent to the contact areas 16 and 17, as measured in across-sectional plane, is extremely acute. Thus wear particles or dirtis easily trapped between the contacting surfaces and burnished intoeither the brush 15 or the ring 11, thereby causing troublesomevariations in the electrical characteristics of the assembly. The spacebetween the brush 15 and the bottom of the groove 12 could, of course,be increased by increasing the diameter of the brush but such a designhas the disadvantage of extending the area of the acute angleintersection surrounding the contact areas so as to create a stilllarger trap for unwanted particles.

In accordance with one aspect of the invention, the sliding contactassembly 20 depicted in FIG. 2 comprises a brush 25 which is wider inone cross-sectional dimension than in the other. 'Brush 25 is positionedin groove 12 with the wider dimension of its cross-section extendingthereacross. Thus, the edges or corners of the brush are in slidingelectrical contact with the walls 13 and 14 of the groove 12 at contactareas 26 and 27. Because of the elongate cross-section of brush 25, itssurfaces in the contact areas 26 and 27 have a relatively high curvaturein a cross-sectional plane. The radii of curvature of brush 25 in thecontact areas is substantially less than one half the maximumcross-sectional dimension of the brush whereas, in the prior artembodiment of FIG. 1, the curvature of brush in the contact areas isequal to one half the maximum cross-sectional dimension, as can be seenfrom the circular cross-section of the brush 15.

By virtue of the relatively short radii of curvature of brush 25 atcontact points 26 and 27, the angles between conductive walls 13 and 14of groove 12 on the one hand and the surface areas of the brush 25adjacent the contact areas on the other hand are substantially lessacute than in sliding contact arrangements of the prior art. Thus, thespace between the brush and the walls of the groove, adjacent to thecontact area, is less constricted. It has been found with thisconfiguration that wear particles, dust and other foreign particles tendto be wiped away from the contact areas instead of being trapped underthem and burnished into the contact surfaces.

The elongate cross-section of brushes employed in the illustrativeembodiments offers an additional advantage over the round cross-sectionof prior art brushes in that a greater maximum cross-sectional dimensionis achieved without adding significantly to the bulk, weight orstiffness of the brush. It is to be noted, moreover, that the relativelyhigh curvature in the contact areas of brushes such as are shown in thedrawing may result in a smaller area contact and consequently a greatercontact pressure 'ice - with the same force applied to the brush.Alternatively,

tional dimension of brush 25, it will ride higher in groove 12 than doesthe round brush 15 illustrated in the embodiment 10 of FIG. 1. A largerspace is therefore left at the bottom of groove 12 for the collection ofwear particles and other undesirable materials which are wiped away fromcontact areas 26 and 27 by virtue of the improved contact configurationof the invention.

Although the invention is illustrated in the embodiment of FIG. 2 with aV-shaped groove or channel it will be appreciated that channels havingother cross-sections which are wider at the top and narrower at thebottom may also be used. Yet other types of channels adapted to receivea brush in sliding conductive contact with the channel Walls may also beemployed, such as channels which vary arbitrarily over their depth froma maximum to a minimum width.

It is a frequent practice in precision sliding contact assemblies toemploy two or more brushes in sliding conductive contact with the wallsof a single groove at areas spaced one from another. For example, abrush and slipring assembly of the type shown in FIG, 1 may employ twobrushes of the same diameter positioned 180 apart in groove 12. Whilesuch a prior art arrangement has the benefit of four contact areas-twoat each brush it is not free of problems since each brush rides onprecisely the same tracks as the other.

In the embodiment of the invention illustrated in FIG. 3, a slip-ring 31having a V-shaped circumferential groove 32 with conductive side walls13 and 14 is contacted by a pair of brushes 35 and 36 having thecharacteristics described above in connection with the brush of slidingcontact assembly 20 shown in FIG. 2. In accordance with the principlesof this invention, however, the longer crosssection dimension of eachbrush dilfers substantially from that of the other. As a result, eachbrush contacts side walls 13 and 14 of groove 12 at a different heightabove the bottom of the groove. Each brush, therefore, rides along aseparate set of tracks so that an accumulation of wear particles orother undesirable material which affects one brush will not affect theother.

Brush contacts side walls 13 and 14 along racks 33 and 34, while brush36 is in contact along tracks 37 and 38. Thus, variations between tracksand/or contacts as a result of variations in the finish, dimensions,compliance 7 or other structural characteristics of the brushes areoffset as each brush creates its own independent set of tracks. Althoughthe assembly of FIG. 3 has two brushes, it will be appreciated that theprinciple of the invention may be extended to a larger number withoutsignificant modification.

Miniature brushes of a type employing the invention may be fabricated byflattening the end portions of the round wire-type brushes usedheretofore. Brushes having different maximum cross-sectional dimensionsmay be made by flattening each brush to a diiferent extent.Alternatively, brushes made from flat stock can be used.

It has been found that a brush and slip-ring assembly as shown in FIG. 3operates with significantly less electrical noise caused by fluctuationsin the contact resistance. Moreover, the amount of wear particlesproduced is substantially reduced, resulting in an extension of theuseful life of the assembly, as well as an improvement of performanceover a substantial portion of the useful life.

In one test, a brush block was fabricated and all brushes were flattenedto give maximum width while keeping the brush thickness within allowablelimits. The brush block was assembled with a slip-ring, subjected to anoil and chlorothene bath and tested for noise, which was found toaverage 26.7 milliohms. The part was then deliberately misaligned tocause wear particles to be produced of a type which, from experience,would increase the noise to a level between 60 and 600 milliohms. Inthis condition the part was rotated for 44 hours, after which the noiselevel was found to have decreased to 18.5 milliohms.

After an additional 24 hours the noise level decreased further to 16.5milliohms. The assembly was then deliberately contaminated with foreignparticles to an extent that could be expected to render the performanceentirely unsatisfactory. The noise was found to have increased only to40 milliohms, and, after 20 additional hours of running, this decreasedto 8 milliohms.

Although the invention has been described with particular reference to anumber of specific embodiments, these are for purposes of illustrationonly. Many variations and modifications in addition to those mentionedherein are possible and may be made by those skilled in this art withoutdeparting from the scope and spirit of the invention. For instance,brushes and grooves of various sizes and cross-sectional configurationsmay be employed to achieve multi-track-contact as taught herein byexample.

What is claimed is:

1. Sliding contact assembly comprising, in combination,

an elongated channel with conductive side walls, said channel beingsubstantially wider at its top than at its bottom, and

a plurality of brushes for engaging said side walls at longitudinallyspaced points in longitudinally sliding conductive contact along aplurality of separate tracks spaced from one another on the walls ofsaid channel, each of said brushes being wider than the bottom andnarrower than the top of said channel.

2. Sliding contact assembly comprising, in combination,

an elongated channel with conductive side walls,

said channel being substantially wider at its top than at its bottom,and

at least a pair of brushes for engaging said side walls atlongitudinally spaced points in longitudinally sliding conductivecontact along at least four separate tracks spaced from one another onthe walls of said channel,

each of said brushes being wider than the bottom and narrower than thetop of said channel and engaging each of said walls along at least onetrack.

3. Sliding contact assembly comprising, in combination,

an elongated channel with conductive side walls,

said channel being substantially wider at its top than at its bottom,and

at least a pair of brushes each having at least a pair of conductivecontact surfaces,

said pairs of surfaces being in longitudinally sliding conductivecontact with said side walls along separate tracks and at longitudinallyspaced points, said tracks being spaced apart on the walls of saidchannel.

4. Sliding contact assembly comprising, in combination,

an elongated channel with non-parallel conductive side walls, and

a plurality of brushes for engaging said side walls at longitudinallyspaced points in longitudinally sliding conductive contact along aplurality of separate tracks spaced from one another on the walls ofsaid channel,

each of said brushes having a cross-sectional dimension greater than thesmallest and smaller than the greatest cross-sectional dimension of saidchannel and engaging each of said Walls along at least one track.

5. Sliding contact assembly comprising,

an elongated V-shaped groove having conductive side walls, and

at least a pair of wire-type brushes mounted for relative movementlongitudinally with respect to said groove,

a portion of each brush being in longitudinally sliding conductivecontact with the side walls of said groove at points longitudinallyspaced from the points of contact of the other brush,

each of said brush portions having an elongate crossscction with thelonger dimension greater than the width of the bottom and smaller thanthe Width of the top of said groove and positioned in said groove withsaid longer dimension extending substantially across said groove,

the longer cross-section dimension of said brush portions differingsubstantially each from the other, each of said brushes being inlongitudinally sliding conductive contact with the side walls of saidgroove along a separate pair of tracks, said pairs of tracks beingspaced from one another on the walls of said grooves.

6. Brush and slip-ring assembly comprising, in combination,

a slip-ring having a brush-receiving circumferential groove,

said groove being substantially wider at its top than at its bottom andhaving conductive side walls, and

a pair of brushes in circumferentially sliding conductive contact withthe conductive walls of said groove at circumferentially spaced points,

each of said brushes being wider in one cross-sectional dimension thanin the other,

the Wider dimension of each brush being wider than the bottom andnarrower than the top of said groove,

each of said brushes being positioned in said groove with its widerdimension of cross-section extending thereacross,

each of said brushes being in circumferentially sliding conductivecontact with the walls of said groove along a separate pair of tracks,

said pairs of tracks being spaced apart on the walls of said groove.

7. Brush and slip-ring assembly comprising, in combination,

a slip-ring having a brush-receiving circumferential groove withconductive side walls,

the width of said groove varying over its depth from a maximum to aminimum, and

a pair of brushes in circumferentially sliding conductive contact withthe side walls of said groove,

each of said brushes having an elongate cross-section with its widerdimension of cross-section being wider than the minimum and narrowerthan the maximum width of said groove,

each of said brushes being positioned in said groove with its widerdimension of cross-section extending between said walls,

each of said brushes being in circumferentially sliding conductivecontact with the Walls of said groove along a separate pair of tracks,

said pairs of tracks being spaced apart on the walls of said groove.

8. Brush and slip-ring assembly comprising, in combination,

a slip-ring having a brush-receiving circumferential groove withconductive side walls,

said groove being substantially wider at its top than at its bottom, and

a pair of brushes each having a pair of curved conductive contactsurfaces,

the radii of curvature of said contact surfaces in a crosssectionalplane being substantially smaller than onehalf the maximumcross-sectional dimension of the respective brush,

said contact surfaces being in circumferentially sliding conductivecontact with separate tracks along the side walls of said groove,

the contact surfaces of one brush being in contact with the walls ofsaid groove at points circumferentially spaced from the points ofcontact of the other brush,

each of said brushes being in circumferentially sliding conductivecontact with each wall of said groove along separate tracks,

said tracks being spaced apart from one another on said walls.

References Cited UNITED STATES PATENTS 2,231,149 2/1941 Baum 33922,581,266 1/1952 Lum 3395 3,155,207 11/1964 Blemly et al. 191-233,259,727 7/1966 Casler 3392 X MARVIN A. CHAMPION, Primary Examiner.

ALFRED S. TRASK, Examiner.

P. TEITELBAUM, Assistant Examiner.

1. SLIDING CONTACT ASSEMBLY COMPRISING, IN COMBINATION, AN ELONGATEDCHANNEL WITH CONDUCTIVE SIDE WALLS, SAID CHANNEL BEING SUBSTANTIALLYWIDER AT ITS TOP THAN AT ITS BOTTOM, AND A PLURALITY OF BRUSHES FORENGAGING SAID SIDE WALLS AT LONGITUDINALLY SPACED POINTS INLONGITUDINALLY SLIDING CONDUCTIVE CONTACT ALONG A PLURALITY OF SEPARATETRACKS SPACED FROM ONE ANOTHER ON THE WALLS OF SAID CHANNEL, EACH OFSAID BRUSHES BEING WIDER THAN THE BOTTOM AND NARROWER THAN THE TOP OFSAID CHANNEL.